Internal fuel staging for improved fuel cell performance

ABSTRACT

A fuel cell having an anode and a cathode side with fuel flowing over the anode side and air over the cathode side has a staging plate located on the anode side of the fuel cell to divide the flow of fuel to two different sections of the anode. A second staging plate may be used to divide the flow of fuel to three different sections of the anode and various apertures may be formed in the plates, such as rectangles triangles or ovals, to direct fuel flow to desired areas of the anode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally drawn to a fuel cell construction foroptimizing fuel cell performance and achieving high fuel cell systemefficiency and more particularly to a staged fuel cell structure forachieving same.

2. Description of the Prior Art

Fuel cells are electrochemical devices that convert the energy of achemical reaction directly into electrical energy. The basic physicalstructure of a single fuel cell includes electrodes (an anode and acathode) with an electrolyte located there between in contact with theelectrodes. To produce electrochemical reaction at the electrode, a fuelstream and a oxidant stream are supplied to the anode and cathode,respectively. The fuel cell electrochemically converts a portion of thechemical energy of the fuel in the fuel stream to electricity, while theremaining amount of the chemical energy is released as heat. A stack ofindividual fuel cells is preferably connected in electrical series togenerate a useful additive voltage.

The type of electrolyte used in a fuel cell is generally used toclassify the fuel cell and is also determinative of certain fuel celloperating characteristics, such as operating temperature. Presentclasses of fuel cells include the Polymer Electrolyte Fuel Cell (PEFC),the Alkaline Fuel Cell (AFC), the Phosphoric Acid Fuel Cell (PAFC), theMolten Carbonate Fuel Cell (MCFC), and the Solid Oxide Fuel Cell (SOFC).

Ideally, fuel cell performance is expected to depend only on the fuelcomposition and the amount of fuel consumed at the anode side. However,typical voltage-current and power characteristics of operating fuelcells show a performance drop due to many resistances, including thefuel utilization resistance. This utilization resistance is primarilycaused by the driving force variation (across the electrode-electrolyteassembly), which is itself due to a fuel composition gradient over theanode surface.

In fuel cell literature, various designs of anode-electrolyte-cathodeand associated flow passages are available for constructing multi-layerfuel cell stacks. The most common configurations are the planar andtubular assemblies. In either case, the fuel and oxidant (e.g., air)flow past the surface of the anode and cathode placed opposite theelectrolyte, respectively, so that the anode surface is in directcontact with the fuel and the cathode surface is in direct contact withair. The flow passages are connected to the inlet and outlet manifoldson both the anode and cathode sides.

In all fuel cells, the fuel composition decreases due to electrochemicalreactions as the fuel passes across the anode from the inlet to theoutlet. This gives rise to species concentration gradients, which aremainly responsible for uneven fuel utilization and unwanted temperaturegradients on the anode surface. The cell voltages drop to adjust to thelowest electrode potential for the depleted species compositions at theexit of the anode and cathode sides.

Referring now to the drawings generally and FIG. 1 in particular, aknown fuel cell assembly (10) is shown. The fuel (4) and oxidant (6),preferably air, flow past the surface of an anode (12) and cathode (14)placed on opposite sides of an electrolyte (not visible) so that theanode surface (12) is in direct contact with the flow of fuel (4) andthe cathode surface (14) is in direct contact with flow of air (6). Theflow passages are fluidically connected to known inlet and outletmanifolds (not shown) on both the anode (12) and cathode (14). Theproblems associated with this type of construction have been describedabove.

Accordingly, staging of fuel cells is one known way to help alleviatethis problem. U.S. Pat. No. 6,033,794 “Multi-stage Fuel Cell SystemMethod and Apparatus” discloses a fuel cell system consisting ofmultiple fuel cells. The gas flow paths in the cells are connected in anexternally staged, serial, flow-through arrangement. This arrangementhas a series of higher temperature fuel cells which utilize theincreased temperature of the fuel as it exits each consecutive fuel cellin order to improve fuel cell efficiency.

Notably, no known staging of the inlet fuel to one individual fuel cellexists, although such inlet staging could provide better utilization ofthe fuel, a more even temperature distribution, and, generally, a moreefficient fuel cell. Thus, inlet staging to a single fuel cell would bewelcome by the industry, as this single cell inlet staging would permitenhanced performance of both individual cells, as well as entire stacks.

SUMMARY OF THE INVENTION

The present invention solves the mentioned problems of improving thefuel and temperature distribution of fuel cells, as well as others, byproviding an internal fuel cell staging technique to alleviate fuelcomposition non-uniformity and the problems associated therewith. Thus,fresh incoming fuel is internally distributed by placing at least oneinternal staging plate inbetween the flow fields of the fuel to theanode of the fuel cell. This plate or plates may be formed as a flowdivider plate having apertures therein (preferably of a rectangular ortriangular shape and/or a pattern of essentially round shapes) to dividethe flow of the raw fuel to different areas of the anode to stage thefuel flow thereby.

In view of the foregoing it will be seen that one aspect of the presentinvention is to provide a single fuel cell with a staged fuel input forincreased efficiency.

Another aspect of the present invention is to provide a uniquedistribution of fuel cell fuel over an anode of a fuel cell.

These and other aspects of the present invention will be more fullyunderstood after a careful review of the following description of thepreferred embodiment when considered with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a cross-sectional side view of a known fuel cell with normalflow of fuel and air through the anode and cathode thereof;

FIG. 2 is a cross-sectional side view of a fuel cell having a stagingplate therein to split the flow of fuel over the anode as per theinvention;

FIG. 3 is a cross-sectional side view of a fuel cell having a pair ofstaging plate therein to split the flow of fuel over the anode as peranother embodiment of the invention;

FIGS. 4a-4 f are perspective views of various types of staging platesfor planar fuel cells with an essentially rectangular shape, wherein theappertures have rectangular and triangular openings therein which may beused in the FIG. 2 or 3 embodiments; and

FIGS. 5a-5 d are perspective view of various types of staging plates forplanar fuel cells with an essentially rectangular shape, wherein theappertures have round and oval opening patterns thereon which may beused in the FIG. 2 or 3 embodiments.

FIGS. 6a and 6 b are top views of various types of staging plates forplanar fuel cells with an disk shape, wherein the appertures have slotsthereon which may be used in the FIG. 2 or 3 embodiments.

FIGS. 7a and 7 b are perspective views of various types of stagingplates for tubular fuel cells, wherein the appertures have slots andround and oval opening patterns thereon which may be used in the FIG. 2or 3 embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 2, where like numerals indicate similar elementsthroughout the drawings, a staged fuel inlet fuel cell assembly (18) isshown to have an internal staging plate (20) located on the anode (12)side of the cell (18). Staging plate (20) may be of any appropriate sizeor spacing from anode (12), as its primary purpose is to split the flowof the fuel (4) into two discrete branches (22, 24) with one branch (22)flowing on top of the plate (20) to react with the downstream side ofthe anode (50) (i.e., the outlet side) while the bottom section (24)flows along the upstream side of the anode (52) (i.e., the inlet side)to react therewith. This split flow minimizes the fuel utilizationresistance by reducing the variation in fuel concentration over theenitre surface anode (12).

With the internal staging provided by the plate (20), the fuel (4) iswell-distributed on different sections of the anode (50, 52).Accordingly, the fuel utilization resistance is lowered and the fuelcomposition gradients are minimized. Not surprisingly, this technique iseven more beneficial for large surface area fuel cells, where the fuelutilization resistance is high.

Referring now to FIG. 3, a second embodiment of the invention is shown.Here, two staging plates (26, 28) are used to further split the fuelflow into three discrete sections (30, 32, 34). As above, the firstplate (26) and second plate (28) are placed strategically to minimizethe fuel gradients across the entire surface of anode (12) and tofurther optimize fuel cell performance. Plate (26) provides a top flowvolume (30) which passes fuel (4) to the downstream portion (50) of theanode (12) while middle flow (32) and plate (28) deliver fuel (4) to themiddle portion (51) and bottom flow (34) is directed onto upstreamportion (52) of the anode (12).

Turning now to FIGS. 4a-4 f, it will be seen that the configuration ofany/all of the plates of the present invention, now generally designatedas (200), may be solid plates or the longitudinal surfaces of these mayhave various outlet configurations formed thereon. These configurationsmay be of any type which help to optimize cell performance, and they aremost preferably in the shape of rectangles (60) (FIG. 4a), squares (61)(FIG. 4e) and/or triangles (62) (FIGS. 4b, 4 c, 4 d, and 4 f) to provideraw fuel to anode areas (not shown) deemed needing staging.

Similarly, FIGS. 5a-5 d show a variety of outlet configurationscomprised of patterns of round, oval, or otherwise curved apertures (63)designed to assist in various types of staging requirements. Theparticular type of opening and configuration will depend on particularcircumstances. However, in the preferred embodiment, the apertures linehave a linear arrangement and cover a rectangular area on the plate(FIG. 5a), a square area on the plate (FIG. 5b), or an essentiallytriangular area on the plate (FIGS. 5c-5 d).

The plates of the present invention (200) must also provide forelectronic conduction. This can be achieved in a number of ways. Theplates can be fabricated from an electronically conductive material,such as high temperature metals or LaCrO₃ type ceramics. Alternatively,the plates are made of an insulating ceramic and electronic conductionis provided by vias (64) filled with a conductive material, as is shownin FIGS. 5a-5 d and 4 a-4 f.

Finally, the present invention is equally applicable to disk-shapedplanar, as well as tubular, fuel cells. FIGS. 6a-6 b show some of thevariations that need to be made to the plate (200) to accommodate suchdisk-shaped planar cells, including at least one slot (64) and a centralaperture (65) for fuel inlet, while FIGS. 7a-7 b cover the variationsattendant to tubular cells, including at least one slot (64). Notably,for tubular arrangements, the plate (200) must also be modified to havethe shape of a tube, such that it will form an annular flow channelaround the anode. As above, these configurations provide the generalframework of the invention, and the exact size and location of theseslots and/or apertures may be varied until the desired performancecharacteristics are achieved.

From the foregoing it will be seen that the present fuel cellconstruction offers certain definite advantages over prior artconstruction as listed below:

1. The proposed internal fuel staging is a novel and economical way toimprove anode side spatial fuel distribution.

2. This technique will improve and minimize the temperature distributionacross the cells.

3. This technique will minimize the fuel utilization resistance byminimizing fuel composition gradients.

4. The staging technique is very simple to implement in multi-layeredfuel cell stacks.

5. The staging plate geometry and other parameters could be optimized togive better fuel cell performance.

6. The staging plates will not complicate the existing flow passages andmanifolds, and will not affect the pressure drops.

7. The plates could be made of the same stack material to match thermalexpansion, electronic conductivity and other properties with those ofthe stack components.

8. The thin plate design will not cause any dramatic increase in stackheight or weight.

9. The technique is equally suited to the cathode-side air staging forincremental benefits.

10. The proposed staging technique could be extended to disk-shapedplanar fuel cell designs, as well as tubular designs.

11. The proposed technique can be applied to other solidelectrolyte-type fuel cells (e.g. PEMs).

Certain additions and modifications will occur to those skilled in thisart area upon considering this disclosure. It will be understood thatall such have been deleted herein for the sake of conciseness andreadability but are intended to fall within the scope of the followingclaims.

We claim:
 1. A fuel cell having an anode side and a cathode side withfuel flowing through the anode side and air flowing through the cathodeside comprising: an anode; an anode chamber defining a path for fuelflow across said anode; and a staging plate located in said anodechamber to divide the flow of fuel there through to different stagingareas of said anode, wherein said anode chamber forms an annular spacesurrounding said anode.
 2. A fuel cell as set forth in claim 1, whereinsaid staging plate is a plate extending parallel to said anode for adistance to divide the fuel flow between a first part of said anode anda second part of said anode.
 3. A fuel cell as set forth in claim 2,wherein said staging plate is formed to have at least one slot over asection thereof.
 4. A fuel cell as set forth in claim 1, wherein thestaging plate includes means for electronic conduction.
 5. A fuel cellas set forth in claim 4, wherein the means for electronic conductioncomprises at least one filled via extending through the staging plate.6. A fuel cell as set forth in claim 1, including a second staging plateand wherein said first and second staging plate divide the flow of fuelacross three different sections of said anode.
 7. A fuel cell as setforth in claim 6, wherein at least one of said first and second stagingplates are formed to have at least one slot over a section thereof.
 8. Afuel cell having a anode side and a cathode side with fuel flowingthrough the anode side and air flowing through the cathode sidecomprising: an anode; and an anode chamber defining a path for fuel flowacross said anode; and a staging plate located in said anode chamber todivide the flow of fuel there through to different staging areas of saidanode, wherein said staging plate has a planar disk shape.
 9. A fuelcell as set forth in claim 8, wherein said staging plate is a plateextending parallel to said anode for a distance to divide the fuel flowbetween a first part of said anode and a second part of said anode. 10.A fuel cell as set forth in claim 9, wherein said staging plate isformed to have a central aperture and at least one slot over a sectionthereof.
 11. A fuel cell as set forth in claim 8, further comprising asecond staging plate located in said anode chamber to divide the flow offuel there through to different staging areas of said anode, whereinsaid first and second staging plate divide the flow of fuel across threedifferent sections of said anode.
 12. A fuel cell as set forth in claim11, wherein at least one of said first and second staging plates areformed to have a central aperture and at least one slot over a sectionthereof.
 13. A fuel cell as set forth in claim 11, wherein the first andsecond staging plates include means for electronic conduction.
 14. Afuel cell as set forth in claim 13, wherein the means for electronicconduction comprises at least one filled via extending through the firstand second staging plates.
 15. A fuel cell as set forth in claim 8,wherein the staging plate includes means for electronic conduction. 16.A fuel cell as set forth in claim 15, wherein the means for electronicconduction comprises at least one filled via extending through thestaging plate.
 17. A fuel cell having an anode side and a cathode sidewith fuel flowing through the anode side and air flowing through thecathode side comprising: an anode; an anode chamber defining a path forfuel flow across said anode; and a staging plate located in said anodechamber to divide the flow of fuel there through to different stagingareas of said anode, wherein said staging plate extending parallel tosaid anode for a distance to divide the fuel flow between a first partof said anode and a second part of said anode and wherein said stagingplate includes at least one filled via extending through the stagingplate.
 18. A fuel cell having an anode side and a cathode side with fuelflowing through the anode side and air flowing through the cathode sidecomprising: an anode; an anode chamber defining a path for fuel flowacross said anode; a first staging plate located in said anode chamberto divide the flow of fuel there through to different staging areas ofsaid anode; and a second staging plate located in said anode chamber todivide the flow of fuel there through to different staging areas of saidanode, wherein said first and second staging plates divide the flow offuel across three different sections of said anode and wherein the firstand second staging plates include means for electronic conduction.
 19. Afuel cell as set forth in claim 18, wherein means for electronicconduction comprises at least one filled via extending through the firstand second staging plates.